In Fallout 3 there is a very clear 'most evil thing in the history ever' moment, but I failed to find such an option in Skyrim.
Thoughts?
I added a poll as a guideline, but please do post if you have your own ideas.
EDIT: I think the evilest thing I can come up with is asking a woman to marry you, arranging a wedding, calling it off at the very last minute, kill all of her friends, then kill the wife, soul trapping her. Then eat her corpse with the Ring of Namira. > : )